A machining center is a computer numerical control (CNC) machining tool with an automatic tool-changing function. The machining center can automatically perform various working such as milling, drilling or notching, boring, tapping, etc. on works set thereto with improved efficiency. A tool holder comprises generally of a tapered portion adapted to be connected to a spindle of a machining center, a manipulator-engaging portion, and a tool holding section for firmly holding the tool.
One type of tool holder is a shrink-fit tool holder, which can firmly hold a tool with excellent dynamic balancing suitable for high-speed work. The shrink-fit tool holder comprises of a tapered connecting portion, a manipulator-engaging portion, and a tool holding section for firmly holding a tool. A shank of the tool is inserted into an aperture of the tool holding section and secured by shrinkage fitting.
The conventional shrink-fit tool holders utilize differences in thermal expansion coefficients between the tool-holding sections and the tool shanks. The tool shanks are made of materials having low thermal expansion coefficients such as sintered carbides, high speed steel, etc., while the tool-holding sections are made of high-expansion materials such as hot work tool steel, nickel-chromium steel, etc.
In these prior art tool holders, the tool shank and the aperture of the tool-holding sections have suitably controlled sizes to achieve detachability, thereby minimizing heating for shrinkage fitting and thus preventing decrease in strength and hardness of materials due to change in their structures. However, in the conventional shrinkage-fit tool holders, the difference in a thermal expansion coefficient between the tool-holding member and the tool shank is insufficient, thereby failing to achieve a sufficient gripping strength.
Accordingly, there is a need for a method and apparatus for retaining a tool in a shrink-fit tool holder by embedding a joining compound in the bore of the tool holder to increase the coefficient of static friction between the tool holder and the tool.